Solid-state image sensors (also known as “solid-state imagers,” “image sensors,” and “imagers”) have broad applications in many areas and in a number of fields. Solid-state image sensors convert a received image into a signal indicative of the received image. Examples of solid-state image sensors include charge coupled devices (CCD), photodiode arrays, and complementary metal-oxide semiconductor (CMOS) imaging devices (also known as “CMOS image sensors” or “CMOS imaging arrays”).
Solid-state image sensors are fabricated from semiconductor materials, such as silicon or gallium arsenide, and comprise imaging arrays of light detecting (i.e., photosensitive) elements (also known as “photodetectors” or “photoreceptors”) interconnected to generate analog signals representative of an image illuminating the device. A typical imaging array comprises a number of photodetectors arranged into rows and columns, each photodetector generating photo-charges. The photo-charges are the result of photons striking the surface of the semiconductor material of the photodetector, and generating free charge carriers (electron-hole pairs) in an amount linearly proportional to the incident photon radiation. The photo-charges from each pixel are converted to a charge signal which is an electrical potential representative of the energy level reflected from a respective portion of the object and received by the solid-state image sensor. The resulting signal or potential is read and processed by video/image processing circuitry to create a signal representation of the image.
In recent years, CMOS image sensors have become a practical implementation option for imagers and provide cost and power advantages over other technologies such as CCD or charge injection device (CID). A conventional CMOS image sensor is typically structured as an imaging array of pixels, each pixel including a photodetector and a transistor region, and as discussed above, each pixel converts the incoming light into an electronic signal.
One type of active pixel design for a CMOS image sensor, often referred to as a pinned-diode pixel, includes four wires (or “metal interconnect lines” or “metal interconnect segments”), a photodetector (i.e. a photodiode), and three transistors, namely a reset transistor, a source-follower transistor, and an access transistor (or “transfer gate”). The photodiode and transistors are located in active areas of a silicon substrate that forms a floor to the pixel. Two of the metal interconnect segments are disposed in a first metal layer (generally referred to as metal-1), which is positioned above a poly-silicon layer formed on the silicon substrate, and provide reset and access (“transfer”) signals to the pixel.
The two remaining metal interconnect segments disposed perpendicularly to the first two metal interconnect segments in a second metal layer (generally referred to as metal-2), which is positioned above a dielectric insulation layer over the first metal layer, and provide power and column selection to the pixel. Conductive contacts couple the metal-1 layer to the poly-silicon layer and to the active areas of the silicon substrate, and conductive vias couple the metal-2 layer to the metal-1 layer. The contacts and via enable the metal interconnect segments to be in electrical communication with one another and with the poly-silicon layer and silicon substrate of the pixel. In a typical three-transistor active pixel design for a CMOS image sensor, each pixel includes four wires (or “metal interconnect lines” or “metal interconnect segments”) and three transistors, namely, a reset transistor, a source-follower transistor, and a select transistor. Two metal interconnect segments are disposed horizontally to provide row selection for either resetting the pixel or reading the pixel. Two other metal interconnect segments are disposed vertically (or substantially perpendicular to the first two metal interconnect segments) to provide column selection for both reading and resetting the pixel.
In conventional CMOS image sensors, the arrangement of the pixel's structures, including the relative positioning of the photodetector, the transistor region, and the metal interconnect segments, as well other structural elements, has presented problems. A major problem which conventional CMOS image sensors exhibit is pixel light shadowing (also referred to as “geometric shadowing”). Pixel light shadowing is caused when the average ray or principal ray striking the pixel deviates significantly from normal (or perpendicular to the imaging array plane). Under these conditions, one or more of the pixel elements situated in metal layers above the photodetector may block a significant amount of light from being directed at the photodetector. As a result, the brightness of the resulting image can be significantly reduced, resulting in poor image quality